把引导程序写进光盘镜像

https://www.jianshu.com/writer#/notebooks/19149135/notes/71282488
从这里我们已经有了boot.bin，且这个boot.bin可以放入软盘中启动虚拟机。
接下来我们研究一下怎么写进光盘，然后用光盘启动虚拟机。

https://stackoverflow.com/questions/34268518/creating-a-bootable-iso-image-with-custom-bootloader
最好的答案出在这里，我们实践一下。

boot.s

.code16
.global init
init:
  mov $0x07c0, %ax
  mov %ax, %ds
  mov $0x07e0, %ax
  mov %ax, %ss
  mov $0x2000, %sp

  mov %cs, %bx
  call print_register

  mov $0x3a, %al
  mov $0x0e, %ah
  int $0x10

  call next
next:
  pop %bx
  sub $(next-init), %bx  # starting point of memory address, now stored in %bx
  call print_register
  jmp .

print_register:  # always print out value in %bx
  mov %bh, %cl
  shr $0x4, %cl
  and $0x0f, %cl
  call print_digit
  mov %bh, %cl
  and $0x0f, %cl
  call print_digit
  mov %bl, %cl
  shr $0x4, %cl
  and $0x0f, %cl
  call print_digit
  mov %bl, %cl
  and $0x0f, %cl
  call print_digit
  ret

print_digit: # %cl has digit to be printed
  cmp $0x9, %cl
  jg print_digit_atof
print_digit_1to9:
  add $0x30, %cl
  jmp print_digit_out
print_digit_atof:
  add $0x57, %cl
print_digit_out:
  mov %cl, %al
  mov $0x0e, %ah
  int $0x10
  ret

.=510
.byte 0x55
.byte 0xaa

把myos.iso放入虚拟机光驱，启动

$ as -o boot.o boot.s
$ ld -o boot.bin --oformat binary -e init boot.o
$ mkdir iso
$ cp boot.bin iso/
$ genisoimage -o myos.iso -b boot.bin -boot-load-size 1 -no-emul-boot -hide floppy.img iso/

参考

https://www.jianshu.com/writer#/notebooks/19149135/notes/71282488
https://stackoverflow.com/questions/34268518/creating-a-bootable-iso-image-with-custom-bootloader